sorry, had to leave for a little bit. the FW dip is the same for seahorses as it is for any other marine fish.
FW dip has to be at same pH and temp as the current tank water. i use RO and buffer accordingly to match current tank peramaters. don't use untreated tap water - chlorines and chloramines are bad (you probably already know that). you're shooting for minutes here, 8-10 is good but depending on the severity of the problem your horse may not tolerate that well. i've gone as high as 12 minutes, key is to keep an eye on him while doing this. sometimes they swim around like they don't have a care in the world, other times they may lay at the bottom of the container. if he does lay down gently tap him. i've never had one of my horses thrash but that is also possible so don't be suprised, you may also see him attempt to scratch his gills with his tail. pull the horse immediately if he really freaks out on you, probably won't, but be prepared.
just to cover all bases here, do not, ever, FW dip a fish with open wounds.
also, ideally i would place the horse in a separate QT tank but you may not be prepared for that but i ask that you consider it. the immediate concern is to give him relief and get him eating again.
you may want to venture over to one of the dedicated seahorse forums and post in their emergency sections. lots of experience there and can help you out as well.
are there any other symptoms other than weak snick? lethargy, heavy breathing, etc. ?
i've got to run for a little bit but will check back when i return.